The present invention relates to a gearbox with six or more forward gears for single-clutch or twin-clutch motor-vehicle transmissions.
A six-gear gearbox of the type specified above is known from European patent application EP-A-1589257 in the Applicant's name. This known gearbox comprises:    a pair of coaxial input shafts, that is, an inner input shaft and an outer input shaft, each of which can be coupled for rotation with a drive shaft by means of a respective clutch,    a pair of output shafts, that is, an upper output shaft and a lower output shaft, which are parallel to the input shafts and are disposed above and below those shafts, respectively,    a first set of driving gearwheels associated with the odd gears (first, third and fifth) and with the reverse gear and carried by the inner input shaft,    a second set of driving gearwheels associated with the even gears (second, fourth and sixth) and carried by the outer input shaft,    a first set of driven gearwheels associated with the second, fourth and reverse gears, which gearwheels are idly mounted on the upper output shaft and can be coupled selectively for rotation with that shaft by means of sliding engagement sleeves, and    a second set of driven gearwheels associated with the first, third, fifth and sixth gears, which gearwheels are idly mounted on the lower output shaft and can be coupled selectively for rotation with that shaft by means of sliding engagement sleeves.
More specifically, to limit the overall axial length of the transmission, in the gearbox that is known from the above-mentioned patent application, a driving gearwheels which is common to the first and reverse gears is provided on the inner input shaft and a driving gearwheel which is common to the fourth and sixth gears is provided on the outer input shaft. These two shared driving gearwheels are disposed at opposite ends of the gearbox. Moreover, the driven gearwheel for the first gear that is idly mounted on the lower output shaft is used as the idle gear of the reverse gear train.
A problem of the known gearbox described above is the considerable length of the upper output shaft which hinders a forward transverse arrangement of the gearbox, particularly with a twin-clutch transmission which itself leads to a greater axial length owing to the presence of a second clutch.